PGMs may occur as discrete minerals or as dilute solid solutions typically in major sulphide minerals (for example, pentlandite, chalcopyrite or pyrrhotite). The separation chemistry of PGMs is amongst the most complex known with treatment being generally more complex as the sulphide or chromitite content of the ore increases. Often, gold is present in minerals rich in PGMs.
Low sulphide PGM ores which contain small amounts of base metal sulphides are typically treated by fine grinding and bulk flotation to give a relatively low-grade PGM concentrate. The flotation reagents used are similar to those typically used for copper and nickel sulphides. The flotation concentrate is then dried before smelting to give a nickel-copper-iron-PGM matte. Smelting is a process by which a metal is separated from its ore in the presence of a reducing agent and a fluxing agent.
The platinum group metals have a greater affinity with sulphide melts than with silicate melts and therefore partition with the matte phase rather than with the slag. The matte is “converted” while molten by blowing air into the matte to oxidise the matte and remove iron and some sulphur. The converter matte is then granulated or allowed to cool slowly so that discrete crystalline phases of nickel sulphide, copper sulphide, and a platinum group metal-containing magnetic phase are formed. This matte is then sent to a base metal refinery where base metals such as copper, nickel and cobalt are removed and recovered by magnetic separation followed by acid leaching, or by direct acid leaching, leaving a high grade PGM concentrate. The high grade PGM concentrate is then sent to a PGM refinery which produces the individual PGM elements in metallic form. This route is expensive and not altogether satisfactory for lower grade sulphide ores.
Medium sulphide ores which contain economic amounts of nickel plus copper (base metals) are typically treated by fine grinding and selective flotation, to give a nickel copper PGM sulphide concentrate. This concentrate is smelted in flash furnaces to give PGM-containing mattes. The mattes are treated in various ways to give nickel and copper metal products plus PGM containing by-products which are sent to a refinery.
High sulphide ores which contain economic amounts of nickel and copper are also typically first treated by fine grinding and selective flotation, with or without magnetic separation, to give separate nickel copper PGM and copper PGM sulphide concentrates. The nickel copper PGM concentrate which is usually low grade is calcined to remove some sulphur and then smelted in reverberatory or flash furnaces as for concentrates from medium sulphide ores.
Such prior art processes may also include gravity concentration in place of or in conjunction with the flotation step. A simplified block diagram of one current process flow sheet is provided in FIG. 1 of the present specification. Recovery of PGMs by gravity methods or by flotation may be difficult for ores with low sulphide mineral content concentration.
Conventional processes suffer from several limitations. Some PGM ores and in particular oxide ores from existing operations cannot be sufficiently upgraded by flotation to produce a concentrate which can be treated by a smelter. The same is often true for high chromitite ores. Power consumption for the total process is high and the smelting process has difficulty in dealing with high chromitite ores, adversely effecting recoveries and costs.
PGM smelting capacity is concentrated in a limited number of countries, particularly South Africa, Canada, USA and Russia. Existing smelters are typically owned by a small number of companies which typically also operate mines associated with the smelters. Moreover, transport of concentrates to the existing smelters is expensive, making projects remote from the existing smelters difficult to establish.
PGM refining capacity is less concentrated than the smelting capacity with numerous independent refineries operating in Europe and Asia in addition to those associated with the operating mines and smelters.
The market for total treatment of PGM concentrates is therefore less competitive than many other metals markets. Smaller projects cannot justify the large capital investment required for a smelter and refinery. There is therefore a need for an improved method for upgrading the PGM concentrates shipped to provide a high grade concentrate which would by-pass the smelter and be able to be shipped direct to a refinery. This would not only decrease the cost of production but increase the competitiveness of the market.
One alternative method to traditional processing that has been suggested in the prior art is selective leaching of PGMs from finely ground ore. There is no accepted solvent system for platinum group metals reported in prior art literature. Bromide, chloride, hydroxide, cyanide, bisulfide, thiosulphate, sulphite, and polysulphide ions and ammonia have all been suggested as suitable ligands for forming complexes with the platinum group metals. However, the stability and low solubility of some of these complexes and their reactivity with gangue minerals in the ore makes some of these ligands unsuitable as lixiviants for platinum group metals.
While PGMs are generally recovered from ores, there is also a significant market for recovery of PGMs from used automobile and other industrial catalysts and from computer and electronics scrap. There remains a need for an improved method of extracting PGMs from source materials other than ores.
It is to be clearly understood that, although prior art techniques are referred to herein, such reference does not constitute an admission that any of these techniques form part of the common general knowledge in the art in Australia or in any other country.
Throughout this specification, including the claims, the words “comprise”, “comprises” and “comprising” are used in a non-exclusive sense, except where the context requires otherwise due to express language or necessary implication, ie. in the sense of “including”.